Lipoxygenase-Mediated N-Demethylation of Imipramine and Related Tricyclic Antidepressants in the Presence of Hydrogen Peroxide

Abstract

In this study, we examined the ability of soybean lipoxygenase to mediate the N-demethylation of imipramine and related drugs in the presence of hydrogen peroxide. Formaldehyde generation resulting from the N-demethylation of imipramine, a prototype drug, was found to depend on incubation time, and the concentration of the enzyme, imipramine, and hydrogen peroxide. Under optimal assay conditions, Vmax values of 14 to 18 nmol formaldehyde/min/nmol enzyme or 133 to 164 nmol formaldehyde/min/mg protein were observed. An inhibition of formaldehyde and desipramine formation by nordihydroguaiaretic acid confirmed the lipoxygenase involvement. The blockade of the reaction by glutathione, dithiothreitol, butylated hydroxyanisole (BHA), and butylated hydroxytoluene (BHT) indicated the generation of a free radical intermediate from imipramine. Desipramine, trimipramine, clomipramine, and diltiazem, but not amitriptyline and doxepin, were also oxidized, albeit at a lower rate. Collectively, the evidence gathered in this study suggests, for the first time, that tricyclic antidepressant drugs may undergo lipoxygenase-catalyzed N-demethylation.